Control system for refrigeration-based compressed-gas dryers

ABSTRACT

Improved control system for refrigeration-based compressed-gas dryers comprising a refrigerating circuit with a compressor ( 10 ), a condenser ( 12 ), a throttling member ( 14 ) and an evaporator ( 16 ), and further comprising a heat exchanger ( 22 ), a condensate separator ( 18 ) and a condensate drain ( 20 ), in which the compressor ( 10 ) is by-passed by a hot-gas valve ( 30 ). Control means ( 38, 40 ) are provided for measuring the temperature of the thermal mass of the dryer, or a temperature correlating therewith, and actuating the compressor ( 10 ) into switching on and off, as well as operating an electromagnetic valve ( 36 ) associated upstream of the hot-gas valve ( 30 ), accordingly, so as to selectively cause the dryer to operate as a hot-gas dryer or a thermal-mass dryer for variable period of times depending on the thermal load.

The present invention refers to an improved control system forrefrigeration-based compressed-gas dryers, i.e. machines that areusually employed for removing moisture contained in a stream ofcompressed gas, in particular compressed air.

A control system for refrigeration-based compressed-gas dryers of theabove-cited kind is described in the publication EP 1 293 243 of thesame Applicant, where a description is given concisely of the variouskinds of dryers that are currently used or can be currently found on themarket.

As generally known in the art, a typical refrigeration-basedcompressed-air dryer usually works to reduce the water-vapour content inthe stream of compressed air by causing the same vapour do condense—as aresult of the air entering the dryer being cooled down—and eventuallyletting off the condensed vapour. In other words, the compressed air isfirst cooled down in an air-to-air heat exchanger (also referred to aseconomizer in the art) by the stream of cold compressed air flowing infrom the evaporator of the refrigerating circuit. The cold compressedair leaving the air-to-air heat exchanger flows into the evaporator ofthe refrigerating circuit, where it cools further down due to theevaporation of a refrigerant or coolant medium. At the evaporatoroutlet, the compressed air reaches its lowest temperature (dew pointunder pressure), to which there corresponds a relative humidity of 100%.Before this air flow is conveyed back into the air-to-air heatexchanger, the condensed water vapour is separated therefrom and letout. At the outlet of the air-to-air heat exchanger, the water vapourcontent remains unaltered (same dew point under pressure), but therelative humidity thereof decreases due to the heat-up effect.

In other kinds of dryers, i.e. the so-called “thermal mass” dryers, useis made of an intermediate medium to cool down the compressed air,wherein this intermediate medium is in turn cooled down in another heatexchanger by the evaporating refrigerant medium.

In the so-called “hot gas” dryers, the refrigeration compressor operatesconstantly, even if there is no flow of compressed air, and therefrigerating output or effect that is produced in excess by therefrigerating circuit is compensated for by the hot gas being pumped bythe compressor, which is by-passed directly on the suction side by acontrol member designed to prevent the temperature of the air fromdecreasing to any value below zero (FIG. 1). This control member istypically a pressure-operated valve controlling the suction pressure ofthe compressor. In these hot-gas dryers, the dew point is practicallyconstant.

In “thermal-mass” dryers, the oscillations, i.e. periodical variationsof the dew point are not negligible and the electric power input isvariable, since it is roughly proportional to the thermal load (acombination of flow rate, temperature, pressure and relative humidity ofthe gas). The refrigeration compressor is controlled by a thermostatthat controls the temperature of the thermal mass (typically atemperature that is correlated to the minimum temperature of the air inthe evaporator). Such thermostat switches the compressor cyclically onand off in order to keep the temperature of the thermal mass withinpre-established limits (FIG. 2). When the compressor is working, therefrigerating power that is produced in excess is stored in the thermalmass and is used to cool down the compressed air when the compressor isswitched off. Dryers of this kind, however, have necessarily largersizes and involve higher costs than the other dryers known in the art,and their power input under nominal conditions, i.e. their power inputrating is higher than that of corresponding hot-gas dryers, owing to thetwofold heat-exchange process that forces the compressor to work atlower suction pressures and, as a result, with a lower energy efficiency(COP).

Unlike thermal-mass dryers, a hot-gas dryer uses heat exchangers (i.e.the air-to-air heat exchanger and the evaporator of the refrigeratingcircuit) that do not have any sufficient thermal mass to keep theperiodical variations of the dew point within an adequately narrow range(e.g., 3° C.) and to ensure a number of on/off cycles of the compressorthat is lower than the highest allowable one prescribed by compressormanufacturers (e.g., 12 cycles/hour).

Therefore, it is a first purpose of the present invention to provide anovel control system for a refrigeration-based compressed-gas dryerthat, although making use of heat exchangers that are typical of hot-gasdryers, is effective in enabling an energy saving effect to be obtainedunder varying conditions of the thermal load, which is rather similar tothe one achievable in thermal-ass dryers.

Another purpose of the present invention is to ensure a lowest possibledew point of the compressed gas, which is further subject to variationsof a satisfactorily limited extent.

Yet another purpose of the present invention is to provide acompressed-gas dryer whose costs are comparably low as the ones ofhot-gas dryers, and is further compact in size and low in weight as ahot-gas dryer.

According to the present invention, these and further aims are reachedin a refrigeration-based compressed-gas dryer provided with a controlsystem incorporating the characteristics and features as recited in theappended claims 1 et seq.

Anyway, features and advantages of the present invention will be morereadily understood from the detailed description that is given below byway of non-limiting example with reference to the accompanying drawings,in which:

FIG. 1 is a schematical view of a compressed-gas dryer of the hot-gaskind with compressor pressure control, according to the prior art;

FIG. 2 is a schematical view of a compressed-gas dryer of thethermal-mass kind with temperature control, according to the prior art;

FIG. 3 is a schematical view of a refrigeration-based compressed-gasdryer incorporating a control system according to the present invention;

FIG. 4 is a diagram plotting the electric power input versus the thermalload in a compressed-gas dryer according to the present invention ascompared with a hot-gas dryer and a thermal-mass dryer according to theprior-art;

FIG. 5 is a diagram plotting compressor operation versus the controlledtemperature, according to the control system of the present invention.

As already noted hereinbefore, and illustrated in FIGS. 1 to 3, arefrigeration-based compressed-gas dryer comprises a refrigeratingcircuit including a compressor 10, which pumps the refrigerant mediuminto a condenser 12, where the same medium is converted from the gaseousphase into the liquid one for flowing then in this state through athrottling member 14 and, finally, into and through an evaporator 16,before being drawn in again by the compressor.

Condensate forming as the gas, i.e. the air is cooled down is removedtherefrom in a condensate separator 18, properly collected and finallylet out by a condensate drain device 20.

In addition, both prior-art dryers and the dryer according to thepresent invention comprise a heat exchanger 22, or economizer, in whichthe compressed gas or air is pre-cooled by the same compressed gas orair undergoing post-heating.

The hot-gas compressed-gas dryer (FIG. 1) also comprises a furtherthrottling member 30, which is usually constituted by apressure-controlled valve that controls the suction pressure of thecompressor by by-passing the latter, so as to prevent the airtemperature from sub-cooling, i.e. decreasing to a value below zero.

As opposed thereto, the thermal-mass compressed-gas dryer (FIG. 2) makesuse of an intermediate compressed-air/liquid heat exchanger 32, asarranged between the heat exchanger 22 and the evaporator 16, and athermostat 34 that typically controls a temperature of the liquid or atemperature at a point of the heat exchanger 32 that correlates to theoutlet temperature of the compressed air from the heat exchanger 32 soas to cycle the compressor 10 on and off in view of keeping thetemperature of the thermal mass within pre-set limits.

According to the present invention, the novel control system for arefrigeration-based compressed-gas dryer is based on the principle ofletting the dryer operate selectively as a hot-gas dryer or athermal-mass dryer, so as to optimize the performance and the efficiencythereof through the measurement and corresponding detection of thethermal load.

The novel control system is first of all aimed at solving the basicproblem of reconciling two mutually clashing technical requirements,i.e. (i) keeping the highest allowable number of on/off cycles per hourof the compressor within the limits prescribed by the dryer manufacturer(e.g., 12 cycles per hour), and (ii) keeping the oscillation orvariation extent of the dew point within limits that are acceptable bythe user (e.g., between 1° C. and 5° C.).

The solution of this problem is based on the consideration that ahot-gas dryer, such as the one described with reference to FIG. 1, has alimited thermal mass, which can however prove sufficient to ensureadequate compliance with the afore-indicated requirements when thethermal load is low (e.g., anywhere between 0 and 5%). In view ofreaching the aims of the present invention, a solution has thereforebeen devised, in which an electromagnetic valve 36 is associatedupstream of the hot-gas bypass device 30 (FIG. 3). This electromagneticvalve 36 has the purpose of allowing the pressure-controlled bypassvalve 30 to work whenever this is necessary, as this shall be explainedin greater detail further on. In this connection, it should be noticedthat the electromagnetic valve 36 is capable of performing itsparticular task even in the case that it is installed downstream of thevalve 30. In a still simpler manner the thermostatic control functionmay be implemented in an electronic control unit 38 of the dryer, dulyconnected with a sensor 40 associated to the evaporator 16 of therefrigerating circuit.

The electronic control unit 38 measures the temperature of the thermalmass of the dryer, e.g. at a point of the evaporator 16 that lies closeto the outlet of the compressed air from the evaporator and/or near theinlet point of the refrigerant medium (with the air and the refrigerantmedium flowing in opposite, i.e. countercurrent directions), so thatthis temperature correlates to the dew point of the compressed air. Inthis case, the sensor 40 is a temperature sensor.

Alternatively, there can be measured the outlet temperature of thecompressed air from the evaporator 16, or the temperature of therefrigerant medium flowing into the evaporator (with the air and therefrigerant medium flowing in opposite, i.e. countercurrent directions).As a further alternative solution, the quantity to be measured may bethe evaporating pressure of the refrigerant medium, since all thesequantities are correlated to the dew point. Anyway, in thelast-mentioned case, the sensor 40 is a pressure sensor.

On the basis of the measured quantity, the electronic control unitactuates the compressor 10 and the electromagnetic valve 36 locatedupstream of the hot-gas bypass valve 30 in the manner described below.

When the thermal load is low (e.g., anywhere between 0 and 5%), thesystem operates as a thermal-mass dryer. In particular, when operatingwith very low loads (i.e. near-zero loads, with practically nocompressed air flowing therethrough), the dryer saves a considerableamount of energy, since its heat losses (brought about by or due to theouter surface of the heat exchangers) are lower than those of a realthermal-mass dryer (with three heat exchangers, instead of only two, anda greater volume due to the means forming the thermal mass).

When the thermal load is on the contrary high (e.g., higher than 20%),the system operates “concurrently” as a hot-gas dryer and a thermal-massone, in that it goes alternately through periods in which it works as ahot-gas dryer and periods in which it on the contrary works as athermal-mass dryer. As the thermal load increases, the periods in whichthe inventive system works as a hot-gas dryer grow longer, while theperiods in which it works as a thermal-mass dryer become shorter. Theelectrical power input increases as the thermal load rises—albeit in anon-linear manner—until it eventually reaches its rated value at athermal load of 100%. This rated value is anyway lower than the powerinput at 100% of the thermal load in a traditional thermal-mass dryer.

This result, which is attainable with the novel control system accordingto the present invention, is clearly inferable from the diagram in FIG.4, where the electric power input of the dryer is plotted against thethermal load. It can be readily noticed that, as compared with athermal-mass dryer, a refrigeration-based compressed-gas dryer providedwith the novel control system of the invention involves a power inputthat, while being higher at “medium” thermal loads (area B in thediagram), is certainly lower at “high” and “low” thermal loads (areas Aand C in the diagram, respectively), wherein such power input is anywaymuch lower than that of a hot-gas dryer at low and medium thermal loads.

When the dryer provided with the inventive control system works in theway of a thermal-mass one, the compressor 10 is normally controlled by asingle-step thermostatic control function, as shown in FIG. 5, where:

TC is the controlled temperature (e.g., the temperature of the thermalmass);

TS is the set point;

ΔT is the differential.

For the insufficiency of the thermal mass of the evaporator 16 to becompensated for, use is made of the hot-gas bypass contrivance 30 byacting on the electromagnetic valve 36 so as to enable the compressor tooperate for a length of time corresponding to the minimum running time(to which there corresponds the highest allowable number of on/offcycles specified by the compressor manufacturer), while preventing icefrom forming in the evaporator to any dangerous or, anyway, unacceptableextent. An example of the above-described logics may be the followingone.

Let it be assumed that the upper limit of hourly on/off cycles of thecompressor is in the number of 12, so that an average time of 5 minuteshas to elapse between two energizations or two de-energizations of thecompressor. Let it also be assumed that the thermal load isapproximately 50% of the rated one (i.e. a condition requiring thehighest number of on/off cycles of the compressor in a thermal-massdryer). Under these conditions, upon the compressor having been switchedoff, i.e. de-energized, the controlled temperature TC quickly reaches upto the value TS+ΔT owing to the small thermal mass being available. Inorder to prevent the dew point from rising beyond the limit ofacceptability, the compressor 10 shall therefore be switched on afterjust quite short a time having actually elapsed from the moment in whichthe same compressor had previously been switched off (such time beingcertainly much shorter than 5 minutes). Once the compressor has beenswitched on, the temperature of the thermal mass TC decreases in anequally quick manner to reach TS before 5 minutes have actually elapsedfrom the last de-energization of the compressor. The control unit 38must at this point actuate the electromagnetic valve 36 upstream of thehot-gas valve 30 to open for the remaining period of time, so as toenable the dryer to operate in the way of a normal hot-gas dryer untilthe required 5 minutes from the last de-energization of the compressorare eventually reached. It is just after this time is reached that thecompressor is switched off. In this way, an adequate dew point isensured along with due compliance with the upper limit required for thenumber of hourly on/off cycles of the compressor, although less energyis saved, actually.

It will usually happen that, at low thermal loads, the electroniccontrol unit 38 keeps the electromagnetic valve 36 associated to thehot-gas valve 30 closed, thereby causing the dryer to work in athermal-mass mode, whereas it will cause the same dryer to work in amixed mode at higher thermal loads.

For the performance capabilities of the dryer to be further improved,and in order to attain a low dew point with just small oscillations orvariations thereof, use is made of a control method based on themeasurement of the thermal load.

For the thermal load to be measured, all it takes is to measure the rateat which the temperature of the thermal mass rises after the compressor10 is switched off. As an alternative option, there can be measured therate at which such temperature decreases after the compressor isswitched on.

Each single dryer, as duly provided with a given compressor 10,air-to-air heat exchanger 22 and evaporator 16, will have rate valuesthat vary depending on the thermal load and are characteristic of eachparticular dryer, wherein such values can be readily and easilydetermined experimentally.

The rate can for instance be calculated as:

the ratio of a pre-set ΔT (temperature difference) to the time telapsing from the moment at which the compressor is switched off to themoment at which the temperature of the thermal mass reaches a valueequal to its value at compressor de-energization plus the pre-set ΔT;

the inverse ratio of a pre-set Δt (time difference) and the differencebetween the temperature measured at the initial instant and thetemperature measured at the final instant of that period of time.

It can be readily appreciated that the greater this ratio, the higher isthe thermal load, and vice-versa.

When these values are known, the compressor 10 and the electromagneticvalve 36 can then be controlled so as to attain the pre-set aims as faras energy saving effect and dew-point control are concerned.

The performance capabilities of the dryer provided with the inventivecontrol system can be still further improved by providing anotherelectromagnetic valve 42 (FIG. 3) upstream of the throttling member 14,so as it closes when the compressor 10 is switched off (to re-open whenthe same compressor is switched on again), thereby preventing therefrigerant medium from flowing from the high-pressure side of therefrigerating circuit to the low-pressure one, especially in the casethat the throttling member 14 is a capillary tube. In fact, such flow ofrefrigerant medium might affect the temperature measurements used tothermostatic control and thermal-load calculation purposes.

1. Improved control system for refrigeration-based compressed-gas dryerscomprising a refrigerating circuit with a compressor (10), a condenser(12), a throttling member (14) and an evaporator (16), and furthercomprising a heat exchanger (22), a condensate separator (18) and acondensate drain (20), in which the compressor (10) is by-passed by ahot-gas valve (30), characterized in that there are provided controlmeans (38, 40) for measuring the temperature of the thermal mass of thedryer, or a temperature correlating therewith, and actuating thecompressor (10) into switching on and off, as well as operating anelectromagnetic valve (36) associated upstream of the hot-gas valve(30), accordingly.
 2. Improved control system for refrigeration-basedcompressed-gas dryers according to claim 1, characterized in that thecontrol means (38, 40) are adapted to selectively cause the dryer tooperate as a hot-gas dryer or a thermal-mass dryer for variable periodof times depending on the thermal load.
 3. Improved control system forrefrigeration-based compressed-gas dryers according to claim 1,characterized in that the control means (38, 40) are adapted to causethe dryer to operate as a thermal-mass dryer when the thermal load islow, and are alternatively adapted to cause the dryer to operate as ahot-gas dryer and a thermal-mass dryer when the thermal load is a highone.
 4. Improved control system for refrigeration-based compressed-gasdryers according to claim 1, characterized in that the control meanscomprise an electronic control unit (38) connected to a sensor (40) thatis associated to the evaporator (16) of the refrigerating circuit. 5.Improved control system for refrigeration-based compressed-gas dryersaccording to claim 4, characterized in that the sensor (40) is atemperature sensor and is adapted to measure a temperature that iscorrelated to the dew point of the compressed air.
 6. Improved controlsystem for refrigeration-based compressed-gas dryers according to claim4, characterized in that the sensor (40) is a temperature sensor and isadapted to measure the outlet temperature of the compressed air from theevaporator (16).
 7. Improved control system for refrigeration-basedcompressed-gas dryers according to claim 4, characterized in that thesensor (40) is a temperature sensor and is adapted to measure the inlettemperature of the refrigerant medium entering the evaporator (16). 8.Improved control system for refrigeration-based compressed-gas dryersaccording to claim 4, characterized in that the sensor (40) is apressure sensor and is adapted to measure the evaporating pressure ofthe refrigerant medium in the evaporator (16).
 9. Improved controlsystem for refrigeration-based compressed-gas dryers according to claim1, characterized in that upstream of the throttling member (14) there isprovided a further electromagnetic valve (42) controlled by theelectronic control unit (38) and adapted to be closed when thecompressor (10) is switched off and to be re-opened when the compressoris started again.
 10. Improved control system for refrigeration-basedcompressed-gas dryers as described and illustrated with reference to theaccompanying drawings, and intended for use in the afore-cited ways.